When there has been a break in an AC power source which energizes a DC power supply, and the AC power source is subsequently restored, there may be a very high current drawn by a DC to DC converter in the DC power supply, particularly if a low impedance load is connected to the converter. Drawing excessive current may result in the destruction of certain components of the DC to DC converter, in particular in the destruction of an input filtering capacitor of the converter. It is therefore important for the drawn current to be controlled so that the DC power supply does not cause too high or too sudden a current drain when it comes on again following either a failure or a transient break in the AC power source.
There are various prior art devices to enable current to be limited in DC apparatus and in particular, in a DC power supply responsive to an AC power source, viz:
1. Arrangements which rely on thermoresistive components, particularly negative temperature coefficient thermistors. In the case of a DC electrical power supply for example, the thermistor is connected to the output of an AC to DC converter in series with the DC to DC converter. The thermistor has a high resistivity at ambient temperature. Following a break in the AC source, there is no current flowing in the thermistor so its resistance increases. When the AC power source is restored, the current drawn by the AC to DC converter is limited by the high resistance value of the thermistor. The thermistor temperature then increases due to the flow of current through it, so its resistance decreases, enabling the current supplied to a load to gradually rise as heat progresses; the resistance continues to decrease until steady state temperature and resistance of the thermistor are reached.
Arrangements which employ thermistors have the disadvantage of heating up as a function of the current supplied by the DC apparatus. Following a break, the resistance of the thermistor may be so low that the component does not sufficiently limit the drawn current. The extent to which the current is limited thus varies as a function of the length of the break.
2. Arrangements formed by a fixed resistor connected in parallel with a relay. These arrangements operate as follows: If there is a break in the AC supply, the relay opens and the current drawn by the power supply is limited by the value of the resistor as soon as the AC supply is restored. By employing suitable control means, the relay is held open until the current supplied by the power supply has reached its steady state value. Once the steady state value has been reached, the relay is closed to short circuit the resistor, whereby the power supply then operates under the normal planned operating conditions. Arrangements which employ a resistor short-circuited by a relay are not very reliable and have a rather long response time.
3. Arrangements formed by a thyristor having a gate electrode driven by an actuating circuit. The actuating circuit causes the thyristor to be fully conductive when the AC supply is on, but gradually increases the conductive properties of the thyristor by altering its firing angle, when the AC source is restored following a break. In this way, the current drawn by the DC power supply is limited and steady state conditions are only arrived at gradually. Circuits having a single thyristor are difficult to operate gradually.